The present invention relates to electrical devices and methods, and in particular to devices and methods for adapting surface mount devices to through hole device applications.
An integrated circuit combines numerous active and passive electrical circuitry elements on a single device called a die or chip. These integrated circuits are interconnected by attaching them to printed circuit boards. Integrated circuits are small and fragile; therefore, for use in mass production they are often sealed in metal packages to protect them from damage. FIG. 1 is a generic illustration of one such metal package 1 of the well-known TO-type having a hermetically sealed can package 3. The integrated circuit is electrically terminated via two or more leads 5 that extend from the package 3, typically in a circular pattern or multiple concentric circular patterns. The leads 5 are used to connect the device to a printed wiring board or a socket.
The wiring on application or parent printed circuit boards include thin metallic signal lines embedded in an insulating material. These signal lines interconnect leads from different circuit packages mounted on the same board. The boards may have several layers of interconnected signal lines to provide all of the required connections. The signal lines route electrical signals among the integrated circuits. The lay out of the signal lines thus determines the placement of the integrated circuit packages on the board. The leads of the integrated circuit packages may connect to the wiring in a variety of ways. One technique includes holes drilled in the board through the wiring at appropriate locations. The leads are inserted through the holes and soldered, whereby mechanical and electrical attachments are made among the leads, the wiring, and the holes.
Another technique is called surface mount technology (SMT). This method includes arranging contact pads on the surface of the printed circuit board. The pads are used to route input/output electrical signals through the leads and the appropriate signal lines in the parent printed circuit board. The package leads may be placed on top of the pads and mechanically and electrically attached by soldering. SMT is widely used for high speed digital communications. Typical SMT packages include plastic leaded chip carriers (PLCC), dual in-line packages (DIP), single in-line packages (SIP), small outline packages (SO), thin small outline packages (TSO), and small outline T-leaded packages (SOT). Each device has a different specific footprint associated with its external leads.
FIG. 2 is a generic illustration of a common SMT integrated circuit carrier 10 of the type that is commonly referred to as a Plastic Leaded Chip Carrier, usually abbreviated as PLCC or PCC. The carrier 10 is a rectangular or square package having a housing 12 with a top surface 14 and four sides 16-19 with I/O connections on all four sides. On the leaded version illustrated in FIG. 2, the I/O connections are a row of uniformly spaced electrically conductive leads 20 extending from each side of the carrier housing 12. The PLCC 10 can have 18 to 100 J-shaped leads. PLCC packages can be either socketed or surface-mounted onto solder pads of a printed wiring board 22. Soldering is preferred for applications requiring long term electrical and mechanical reliability.
Over time the design of SMT integrated circuits, or ICs, has improved functionality, which has resulted in smaller devices with fewer I/O connections required to perform the same tasks more efficiently than the older TO-type packaged devices. The carriers, such as the PLCC 10 illustrated in FIG. 2, have evolved to replace the older TO-type packaged devices. According to one example, one or more known ICs packaged in a TO-type package has become obsolete through having its functionality provided more efficiently in a newer IC that is packaged in a more compact SMT device having a smaller foot print.
New printed wiring board designs reduce the space available to match the more compactly packaged SMT device and provide surface-mount solder pads in an appropriate footprint. However, many pre-existing printed wiring board are designed for use with the now superceded or obsolete TO-type packaged devices. While an SMT-type replacement device is functionally equivalent and conveniently smaller, these earlier designs will not accept the newer SMT-type device. Some high-production boards have been redesigned and re-laid out to accommodate the new smaller SMT-type devices having a smaller foot print. However, board redesign requires considerable time that impacts production schedules and involves financial investments that may not be easily justified on legacy products with a limited future.
Rather than bear the disruption and expense of a redesign, some of these legacy products use an adapter that accommodates the difference in footprints and technology between the TO-type and SMT-type devices.
Unfortunately, known adapters for TO-type and SMT-type devices are limited.
The present invention overcomes limitations of the prior art by providing an electrical device adapter that adapts a surface mount technology (SMT) device to through hole applications, whereby a superceded or obsolete electrical device housed in a conventional TO-type device package is replaced by a functionally equivalent and conveniently smaller SMT device.
Accordingly, the present invention provides an electrical device adapter that includes: a printed wiring board having at least a circuit layer and a ground layer between opposing primary and secondary sides; a plurality surface mount technology (SMT) device solder pads arranged on the primary side of the printed wiring board to accommodate a plurality of input/output signal pins of a SMT device; a plurality of plated vias electrically coupled between the solder pads and the circuit layer of the printed wiring board; a first plurality plated through holes extending between the primary and secondary sides and being electrically interconnected to the circuit layer of the printed wiring board, different pairs of the first plurality of plated through holes being structured to accommodate different discrete electrical components; a second plurality plated through holes extending between the primary and secondary sides and being electrically interconnected to the circuit layer of the printed wiring board, the second plurality of plated through holes being arranged in a pattern structured to imitate leads of a conventional TO-type device package; and an elongated input/output signal interconnect pin soldered in two or more of the second plurality plated through holes and extending a distance from the secondary side of the printed wiring board.
According to another aspect of the invention, the a plurality of plated vias of the electrical device adapter that are electrically coupled between the solder pads and the circuit layer of the printed wiring board are embodied as a third plurality of plated through holes extending between the primary and secondary sides and being electrically interconnected to the circuit layer of the printed wiring board.
According to another aspect of the invention, the second plurality of plated through holes of the electrical device adapter form a circular pattern.
According to another aspect of the invention, the printed wiring board of the electrical device adapter includes only a single circuit layer and a single ground layer between the primary and secondary sides.
According to still another aspect of the invention, the electrical device adapter is embodied to replace an obsolete oscillator circuit housed in a TO-type device package with one or more SMT devices and one or more discrete components, whereby a signal input to the electrical adapter via one or more of the interconnect pins equivalent to input signal leads of an oscillator circuit housed in a TO-type device package generates an output signal on one or more different ones of the interconnect pins equivalent to output signal leads of an oscillator circuit housed in a TO-type device package that is equivalent to an oscillator circuit housed in a TO-type device package.